1. Field of the Invention
The present invention relates to an eddy-current probe that is able to detect object's shapes, defects and so on nondestructively.
2. Description of the Related Art
Eddy-current testing (ECT) technique is frequently utilized for nondestructive testing of distorted surfaces of important metal machine parts used in a nuclear power plant, an airplane and so on, such as turbine blades, various pipes and airplane wings. Generally, such an ECT probe using the eddy-current includes mainly an exciting coil and a detector coil for detecting a magnetic field based on an eddy-current induced by an alternating magnetic field generated by the excited coil. Such a technique is described in for example, Japanese Patent Publications Nos. 07-083884A, 09-189682A, 11 -248685A and 2002-090490A.
Further, a planar-type ECT probe for inspecting printed circuit boards is proposed, including a meander-type exciting coil and a pick-up coil for the eddy-current detection which are formed on a flexible planar substrate. Such a probe is described in for example, T. Miyagoshi, D. Kacprzak, S. Yamada and M. Iwahara, “Feasibility of Inspecting Defects in Printed Circuit Boards by Using Eddy-Current Testing Techniques”, Journal of the Magnetics Society of Japan, Vol. 23, No. 4-2, pp. 1613-1616, 1999, and S. Yamada and M. Iwahara, “Trend of Detection Techniques Using Planar-Type Micro-Eddy-Current Testing Probes”, Journal of the Magnetics Society of Japan, Vol. 23, No. 7, pp. 1817-1825, 1999.
Recently, in such an ECT probe, an element for detecting the eddy-current, that is, an eddy-current sensor has been intended to be miniaturized, and to be improved in resolution and sensitivity. In order to improve its detecting resolution, as well as to miniaturize it, the ECT probe has been required to have less spacing between the sensor and a subject.
It is difficult for the planar-type ECT probe using a planar substrate to constantly keep the spacing between the surfaces of the substrate and of a subject much small. In some cases, the surfaces of the substrate and of the subject are almost in contact with each other. Further, when the subject has distorted surfaces, the ECT probe using a flexible thin substrate is desirable to be utilized to follow the surfaces smoothly. However, it is impossible to follow such a flexible substrate in no contact with the subject's surface.
When the surfaces of the substrate used in the planar-type ECT probe and of the subject are almost in contact with each other, an adsorption phenomenon (sticktion) is likely to occur between the surfaces of the substrate and of the subject.
When the sticktion occurs, some external-force application is needed to remove the probe substrate from the subject's surface against the sticktion. The application is likely to damage the probe substrate. The weaker is the strength of the substrate, the damage by the sticktion occurs more frequently. Because the flexible substrate has a small thickness and a weak mechanical strength, the durability and lifetime of the planar-type ECT probe depend largely on the occurrence of the sticktion, especially in the measurement of the distorted surface where the substrate inevitably has a contact with the subject's surface.
This problem tends greatly to appear in micro-defect detection on the smooth surface of the substrate.